Typically, a direct conversion FM receiver converts a received FM signal into a baseband (zero-IF) signal by mixing the received FM signal with a local oscillator (LO) having a frequency substantially equal to the carrier frequency. The baseband signal is then demodulated to recover modulation information. A direct conversion FM receiver may implement the mixing process in one stage, single conversion, or in multiple stages, such as dual conversion.
Single conversion zero-IF receivers have several inherent problems, such as local radiation resulting from the mixing process, desensing of nearby receivers, desensitization by DC offsets or carrier attenuation when AC coupling is used, generation of beat notes if the baseband paths are imperfectly matched, and not providing means for adequately limiting the baseband signal. The use of a dual conversion receiver eliminates several of the inherent problems of the single conversion receiver but still does not provide means for adequately limiting the baseband signal, does not eliminate the desensitization by DC offsets, nor does it eliminate the generation of beat notes.
There are several solutions to the above mentioned problems of the dual conversion receiver. For example, U.S. Pat. No. 4,653,117 assigned to Motorola, Inc., discloses an analog solution, U.S. Pat. No. 4,733,403, assigned to Motorola, Inc. and issued patent application Ser. No. 06/890,804, assigned to Motorola, Inc., disclose digital solutions, and U.S. Pat. No. 4,464,770, discloses an analog-digital solution. The analog solution of patent '117, discloses a dual conversion FM receiver using phase locked direct conversion IF that up-converts the baseband signal to a non-zero IF for amplification, limiting and demodulation. The disclosed FM receiver comprises a quadrature baseband mixing section, followed by an up-conversion section which also amplifies and limits the up-conversion signal, and a phase lock section which precisely centers the zero-IF signal and demodulates the received signal. The gain of the circuit which centers the zero-IF signal (low frequency path) and the demodulation gain (high frequency path) must match to obtain a flat baseband response, thus, component tolerances are critical. Even with tight component tolerances, manufacturing adjustments are usually required to obtain a flat baseband response.
The digital solution of patent '403, discloses a digital zero-IF selectivity section which operates on a recovered input signal. The disclosed invention eliminates the needed up-conversion of the analog solution and improves the selectivity of a conventional dual conversion receiver, but relies on external circuitry to provide the input signal in a digital format. Typically, the external circuitry must operate at a very high sampling rate and is relatively expensive in comparison with analog mixers. The digital solution of issued patent application Ser. No. 06/890,804 discloses a digital radio frequency (RF) receiver which is completely implemented with digital circuitry. The digital RF receiver eliminates the problems of a conventional dual conversion receiver and overcomes several problems of digital receivers such as high sampling rates, resolution in A/D steps, and receiving low level RF analog signals. Nevertheless, at present, the disclosed digital RF receiver is relatively more expensive than a completely analog receiver.
The analog-digital solution disclosed in patent '770 discloses a radio receiver with an analog high frequency single conversion section followed by a digital low frequency section. This invention overcomes the need for high speed digital circuitry, thus reducing the cost, but does not mention overcoming some of the above mentioned inherent problems of a single conversion receiver and further depends on imprecise analog components to determine the recovered baseband response.
Therefore, a need exists for an FM receiver which utilizes the benefits of digital circuitry as well as analog circuitry while overcoming the problems of each implementation.